Voltage-controlled ring oscillators are widely used in numerous applications. A voltage-controlled ring oscillator comprises a plurality of stages of voltage-controlled delay cells (VCDC) configured in a ring topology, wherein each one of said stages of voltage-controlled delay cells receives an input from a preceding stage and outputs an output to a succeeding stage, and a circuit delay from the input to the output is controlled by a control voltage. FIG. 1 depicts a 3-stage voltage controlled ring oscillator 100 comprising three voltage controlled delay cell (VCDC) 110, 120, and 130; each VCDC, configured in a balanced (i.e., differential) circuit topology, has a first (or positive) input terminal Vi+, a second (or negative) input terminal Vi−, a first (or positive) output terminal Vo+, a second (or negative) output terminal Vo−, and a control terminal VC; an input is defined as a voltage difference between the two input terminals Vi+ and Vi−; an output is defined as a voltage difference between the two output terminals Vo+ and Vo−; and a circuit delay from the input to the output is controlled by a control voltage applied at the control terminal VC. A control voltage VCTL is applied to all three VCDC 110˜130; the control voltage VCTL determines the circuit delay of the three VCDC, and therefore determines an oscillation frequency of the ring oscillator. The circuit delay of each VCDC contributes to a phase shift to the oscillation signal.
To sustain the oscillation, the total phase shift must be 360 degrees when the oscillation signal traverses along the ring and returns to the starting point. To assist the oscillation, a polarity inversion is employed in the ring to introduce a 180 degrees phase shift, so that the requirement of phase shift from the circuit delay of the ring to sustain the oscillation is reduced to 180 degrees. In the three-stage voltage-controlled ring oscillation 100 of FIG. 1, the polarity inversion is employed between the output of VCDC 130 and the input of VCDC 110; to sustain the oscillation, each of the three VCDC needs to contributes 60 degrees of phase shift. Besides the requirement on phase shift, each VCDC also needs to provide a gain for the ring to sustain the oscillation. There are many circuits suitable for embodying a voltage controlled delay cell. In order to provide the gain, a voltage controlled delay cell must include an amplifier circuit. To sustain a high frequency oscillation, the delay of the amplifier circuit must be small and therefore the amplifier must be a high speed amplifier. In general, a high speed amplifier comprising MOS (short for metal-oxide semiconductor field-effect transistor), resistors and/or capacitors contribute to no more than 90 degrees of phase shift. To have 180 degrees of phase shift, at least two stages are needed. Therefore, prior art voltage controlled ring oscillators comprise at least two stages of voltage controlled delay cells. Generally speaking, a single-staged oscillator is feasible only if an inductor is employed to achieve 180 degrees of phase shift. Inductors, however, are expensive and not attractive in a cost sensitive design.
What is desired is a single-staged balanced-output oscillator without using inductor.